Sketch-based abstraction for character posing and synthesis

ABSTRACT

In particular embodiments, a 2D representation of an object may be provided. A first method may comprise: receiving sketch input identifying a target position for a specified portion of the object; computing a deformation for the object within the context of a character rig specification for the object; and displaying an updated version of the object. A second method may comprise detecting sketch input; classifying the sketch input, based on the 2D representation, as an instantiation of the object; instantiating the object using a 3D model of the object; and displaying a 3D visual representation of the object.

PRIORITY

This application claims the benefit, under 35 U.S.C. § 119(e), of U.S.Provisional Patent Application No. 62/170,458, filed 3 Jun. 2015, whichis incorporated herein by reference.

TECHNICAL FIELD

This disclosure generally relates to animation tools.

BACKGROUND

In traditional two-dimensional (“2D”) animation, artists may craft eachpose of a character by hand using pencil and paper or any other suitablemedium. This tangible connection may provide a powerful interface thatmay give artists substantial control over a character's shape. Inthree-dimensional (“3D”) animation, posing a character may be a moreinvolved endeavor, since it may entails the coordinated movement ofthousands or more vertices. To make this process tractable, riggingartists may build rigs that may define the space of meaningfuldeformations for a character in terms of abstract rigging parameters(e.g., a full character rig or a rig for a portion of a character). Insome situations, animators may determine a character's pose indirectlyby choosing values for these parameters.

Rigging is the process of taking a static 3D mesh and preparing it foranimation. This process involves creating a hierarchical set ofinterconnected “bones” (referred to as the skeleton or character rig)which is then used to animate the mesh. Additionally, rigging acharacter may involve placement of the character rig (or the skeleton),creation of a joint hierarchy, forward kinematics, inverse kinematics,defining degrees of freedom and constraints, and/or facial rigging. Inreference to the joint hierarchy, in order for a character rig tofunction properly the joints must follow a logical hierarchy. Forexample, the first joint created in the character rig is called thejoint root and thereafter every subsequent joint may be connecteddirectly or indirectly to the joint root. Forward kinematics (FK) refersto one of the basic ways to calculate the joint movement of a fullyrigged character. For example, moving a character's hand may involve themovement of not only the character's hand, but also the character'sshoulder, elbow, etc. With inverse kinematics (IK), degrees of freedomand constraints are used to ensure that a characters rig stays withinthe bounds of nature, that is, not allowing a character head to rotate afull 360 degrees. Lastly, facial rigging may include an entirelyseparate rig defined solely for the face of a character.

From the artist's perspective, crafting by hand or 3D animation mayoffer potential tradeoffs in terms of the creative process. In thehand-drawn case, artists may have substantial, tangible control overmost aspects of a character's shape. However, a great deal of skill maybe required to draw a character's proportions appropriately.Additionally, an extra clean-up process may be needed to ensure that thedrawings are “on model.” “On model” refers to the concept of confirmingthat the artists sketch of the character's shape depicts a single unity.In 3D animation, rigging may naturally encapsulate the space ofmeaningful shapes, potentially obviating the need for the clean-up step.In order to accommodate the broad range of expressive deformation, aproduction character rig may employ hundreds, thousands, or moredifferent rigging controls, which may vary in complexity from blendshapes to skeletal deformation to complex procedural functions.

SUMMARY OF PARTICULAR EMBODIMENTS

In particular embodiments, a sketch-based abstraction method may enablesketch-based posing of 3D characters or the design and customization of3D characters.

In particular embodiments, a method augments a 3D character with a 2Drepresentation and provides a general-purpose energy formulation thatmeasures the distance between the character's 2D representation and auser-drawn 2D sketched curve. By minimizing this energy with the riggingcontrols of the character as unknowns, the system may provide a bridgebetween 2D sketches and the 3D model deformed by the same rigparameters.

The method may be applied to different posing tasks and may also be usedto create customized character designs. In particular embodiments themethod may be applied to mechanical assemblies. In particularembodiments, the character rig's subspace may include the entirecharacter rig as defined by the initial rigging of the character. The 2Drepresentation may be created on the fly by sketching on top of thecharacter's mesh. In other embodiments, the 2D representation may bebuilt directly into the character rig as a set of rigged curves duringcharacter design. In particular embodiments, the character rig describesall of the rigging parameters and features of a particular character.

In particular embodiments, once the 2D representation has been created,user input in the form of a new sketched 2D curve may be received, andthen an energy formulation may be used that measures the distance fromeach of a set of points in the character's 2D representation to arespective set of corresponding points in the new 2D curve. Thealgorithm may minimize the energy using the character's rigging controlsas unknowns. Since the rigging may influence both the 3D shape and the2D representation, the algorithm provides a bridge between the 2D worldof sketching and the character's 3D rigging controls. By minimizing theenergy metric, the described embodiment may deform the 3D character.Therefore, the character rig may be treated as a black-box system, thatis, in particular embodiments, the method may enable arbitrary riggingcontrols, including but not limited to, skeletal deformations, blendshapes, and arbitrary nonlinear procedural functions.

Applications of the method may include draw-over posing for an entirecharacter or for components thereof, creation of characters andcharacter components, and manipulation of non-intuitive subspacesdefined by the constraints of mechanical assemblies.

The embodiments disclosed above are only examples, and the scope of thisdisclosure is not limited to them. Particular embodiments may includeall, some, or none of the components, elements, features, functions,operations, or steps of the embodiments disclosed above. Embodimentsaccording to the invention are in particular disclosed in the attachedclaims directed to a method, a storage medium, a system and a computerprogram product, wherein any feature mentioned in one claim category,e.g. method, can be claimed in another claim category, e.g. system, aswell. The dependencies or references back in the attached claims arechosen for formal reasons only. However any subject matter resultingfrom a deliberate reference back to any previous claims (in particularmultiple dependencies) can be claimed as well, so that any combinationof claims and the features thereof are disclosed and can be claimedregardless of the dependencies chosen in the attached claims. Thesubject-matter which can be claimed comprises not only the combinationsof features as set out in the attached claims but also any othercombination of features in the claims, wherein each feature mentioned inthe claims can be combined with any other feature or combination ofother features in the claims. Furthermore, any of the embodiments andfeatures described or depicted herein can be claimed in a separate claimand/or in any combination with any embodiment or feature described ordepicted herein or with any of the features of the attached claims.

The foregoing description of the preferred embodiments of the presentdisclosure has been provided for the purpose of illustration anddescription. It is not intended to be exhaustive or to limit theinvention to the precise forms disclosed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an example method of a sketch-based abstraction ontop of arbitrary rigs.

FIG. 2A illustrates an example of the draw-over posing embodiment usinga production quality character rig based on a skeletal structurefocusing on the region of the head.

FIG. 2B illustrates an example of the draw-over posing embodiment usinga production quality character rig based on a skeletal structurefocusing on the right leg of the elephant.

FIG. 2C illustrates an example of the draw-over posing embodiment usinga production quality character rig based on a skeletal structurefocusing on the left leg of the elephant.

FIG. 2D illustrates an example of the draw-over posing embodiment usinga production quality character rig based on a skeletal structurefocusing on the tail of the elephant.

FIG. 3 illustrates an example of face-posing using the draw-over posingembodiment using a facial rig based on a large set of blend shapes.

FIG. 4 illustrates an example of redraw posing by embedding the 2Drepresentation.

FIG. 5 illustrates an animation created using redraw posing.

FIG. 6 illustrates an example of sketch-based characterindividualization.

FIG. 7 illustrates an example of the different results achieved throughsketch-based character individualization.

FIG. 8 illustrates a mechanical leg and its corresponding 2Drepresentation with accompanying motion curve.

FIG. 9 illustrates an example method of creating mechanical assemblies.

FIG. 10 illustrates an example of making fine-scale adjustments to acharacter rig.

FIG. 11A illustrates a flowchart for an example method of sketch-basedposing of 3D characters.

FIG. 11B illustrates a flowchart for an example method of design andcustomization of 3D characters.

FIG. 12 illustrates an example computer system.

DESCRIPTION OF EXAMPLE EMBODIMENTS

Particular embodiments provide sketch-based abstraction method thatworks on top of arbitrary rigs, allowing sketch-based posing and/or thedesign and customization of novel creatures. Generally, the embodimentdescribed herein relates to a sketch-based abstraction on top ofartist-designed subspaces. In particular embodiments, in order tocomplement and enhance existing production workflows, embodimentsdescribed herein may focus on 3D character models created and riggedusing standard animation software.

For purposes of illustration, multiple examples of the sketch-basedabstraction method are disclosed. These examples include, but are notlimited to, draw-over posing, redraw posing, sketch-based characterindividualization, and sketch-based design of mechanical characters. Asused herein, the term “sketch” may refer to a stick-figure-style drawingof a character or portions thereof; the sketch may be received as userinput by way of any appropriate input device, such as, for example, anelectronic, mechanical, or capacitive stylus used with a touch pad/touchscreen.

With respect to embodiments providing methods of draw-over posing, auser may draw a sketch on a character (either the entire character or aportion thereof) and then redraw the sketch in the desired posedposition. For purposes of illustration herein, with respect to thedraw-over posing embodiment, the user's first drawn sketch shall bereferred to as the source sketch and the user's redrawn sketch shall bereferred to as the target sketch. In particular embodiments, as a resultof the draw-over posing, the character's rig may be automaticallyupdated to optimally match the two sketches. Draw-over posing may beused for, by way of example but not limitation, skeletal posing, curvedeformers, facial blend shapes, and other more elaborate deforms thatare not typically accommodated by inverse kinematics systems.

In particular embodiments providing methods of redraw posing, a user mayconstruct the 2D character representation by embedding curves in thecharacter's structure and rigging them alongside the character's mesh.Therefore, at runtime a user need only draw the target sketch.

With respect to embodiments providing methods of creating sketch-basedcharacter components, the sketch-based abstraction may be applied toindividual components of a character, wherein the individual componentsmay be authored separately. In particular embodiments, for example, auser may create a variety of custom characters by sketching individualcomponents such as body segments, legs, heads, wings, and tails. Becausethe user may have the ability to define the appropriate 2D abstractionfor each component, the 2D representation may be much simpler than theactual 3D shape. Therefore, a simple pictorial sketch may result in acomplex character design.

Furthermore, particular embodiments may provide methods of manipulatingnon-intuitive subspaces defined by the constraints of mechanicalassemblies.

FIG. 1 illustrates an example pipeline for a method for creating andutilizing a sketch-based abstraction on top of a rig. The rig may have abroad range of expressive deformations and may employ hundreds orthousands of rigging controls, which may vary in complexity from blendshapes to skeletal deformations to complex procedural functions. A 3Dmodel's vertices may define the space of all possible deformations,while the rig defines the subspace of meaningful ones. In order toaccommodate the full range of expressive deformation for a character ora portion of a character, the corresponding rig may employ a range ofcontrols, including, but not limited to, skeletal kinematics, blendshapes, or arbitrary nonlinear procedural deformations. In particularembodiments, the system may treat the rig as a black box, taking thecharacter mesh and a list of rig parameters as input.

The method may begin in an input creation phase 100, followed by runtimephase 110. Input phase 100 may comprise creation of an asset, in which arig (parameterized 3D model 120) forms the basis for creating a designsubspace 130 (2D representation of the rig) by projection into thecamera's viewing plane. In particular embodiments, once the systemcontains the parameterized model 120, in order to provide a connectionbetween the character's rigging controls and the 2D sketches, the systemmay enhance the classical rig parameterization with a 2D representationof the model that may be deformed by the same controls as the surfacemesh. This extended mapping may be referred to herein as a “designsubspace.”

In a first step during runtime phase 110, once user input in the form of2D sketch 140 is received, a matching energy optimization may beperformed (step 150), in which a correspondence between the 2D sketch140 and the 2D representation of the rig is determined. In a second stepduring runtime phase 110, the optimized parameters for the 2Drepresentation are then applied to the rig, resulting in posed 3D model160.

The step 150 of performing the matching energy optimization may begin bygenerating or accessing the parameterized 3D model 120, which maycontain a surface mesh with n vertices x=(x₁; x₂; . . . ; x_(n))^(T).The rig may define an abstract mapping from a set of rig parameters p toa corresponding surface deformation x(p). Enhancing the rig with a 2Drepresentation of the model 120 consisting of k points z=(z₂; z₂; . . .; z_(k)), the system may generate the extended mapping: p→{x(p), z(p)}.In particular embodiments, the specific choice of mapping from theparameters p to the 2D points z may depend on the application.

Once the system has enhanced the classical rig parameterization with a2D representation of the model, that is, design subspace 130 has beendefined, the user may then sketch input to achieve a new pose. Toachieve a new pose a user may first draw the source sketch, indicatingthe region of the character to move, and then draw the target sketch,indicating where the user would like the target sketch to move to inorder to achieve a new pose.

In particular embodiments, a user may not need to first sketch on thecharacter, the user may build a 2D representation directly into the rig.This procedure allows the user to define a “2D language” for posing. Inparticular embodiments, the system may automatically determine theoptimal pose in response to the user-specified 2D representation.

In particular embodiments, the 2D representation may be created on thefly by drawing a curve onto the surface of the character which is thencarried along as the character deforms. Alternatively, the user mayembed a set of curves into the character itself and may rig themalongside the character's mesh. A projection into the camera's viewingplane may yield a 2D representation of the character's current pose.

In particular embodiments, once the rig may simultaneously influenceboth the 3D character and the 2D representation, the system may connectthese two representations via the rig, allowing the system to controlthe character's 3D shape by minimizing a 2D energy based on sketchmatching. Given a new 2D sketch, the system may define an optimizationproblem in the form of a nonlinear iterative closest point (hereinafter“ICP”) objective that may attempt to align the character's 2Drepresentation to match the user defined sketch.

In particular embodiments, optimization may be defined over the rigparameters, thereby minimizing the ICP energy that compares 2D objects.Minimizing the ICP energy that compares 2D objects may also deform the3D shape to match the sketch. An application-dependent regularizationterm may be used to resolve ambiguities in a way appropriate for eachparticular application.

In particular embodiments, taking a sampling of the input strokeprovides a set of 2D points of size m given as y=(y₁, y₂; . . . ; y_(m))that may be used to match the model's 2D representation z. An ICP-basedapproach may be used to perform the matching energy optimization:W _(match)(ωz(p))=Σ_(i=1) ^(m)Σ_(i,j=1) ^(k)ω_(i,j) ·∥y _(i) −z_(j)(p)∥₂ ²  (Eq. 1)

where ω_(i,j) denotes the correspondence weight between points y_(i) andz_(j) that associates them with each other. To minimize W_(match), thesystem may then alternate between fixing p (and thus z) to computecorrespondence weights ω, and fixing ω to optimize for p.

In particular embodiments, a selection of one of two correspondenceprocedures may be predefined during the asset creation phase. The firstcorrespondence procedure may perform an arc-length parameterizedresampling using the same number of sample points for both user sketch140 and the model's 2D curve representation. One-to-one correspondencesbetween y and z may be established for ω_(i,j) by considering thedrawing direction of both lines. The second correspondence procedure maybe selected for more complicated gestures, and it may be based onequidistant sampling of both user sketch 140 and the model part's 2Drepresentation. This may result in sets of points of different sizes.Therefore, to handle the multiple sets of different points, computingmany-to-many correspondences between y and z to minimize W_(match) withrespect to ω may be required using a greedy two-step approach.

The first step may require computing one-to-many correspondences from yto z that may be called forward correspondences a_(i,j)∈{0, 1} byassigning the closest match z to every point y_(i). The second step mayrequire analogously computing backward correspondences b_(ij)∈{0, 1}from z to y and then combining them to final correspondence weights asω_(i,j) =a _(i,j) +b _(i,j).  (Eq. 2)where ∀∈Σ_(i=1) ^(m) a _(i,j)=1 and ∀∈Σ_(i=1) ^(k) b _(i,j)=1.  (Eq. 3)

After determining the correspondence weights, the system may minimizethe matching energy 150 W_(match) by modifying the parameters p thatcontrol the 2D representation points z of design subspace 130. Inparticular embodiments, given that the correspondence weights ω arefixed, the optimization problem:

$\begin{matrix}{{\arg{\min\limits_{p}\mspace{14mu}{W_{match}( {\omega,{z(p)}} )}}},} & ( {{Eq}.\mspace{11mu} 4} )\end{matrix}$

may be solved using a Newton-Raphson scheme. To prevent the costlycomputation of higher-order derivatives, a method may be employed tolinearly approximate the subspace mapping asz(p)≈z(p ₀)+J(p ₀)·(p−p ₀),  (Eq. 5)

where it can then estimate the Jacobian

$J = \frac{\partial z}{\partial p}$using finite differences around the initial parameter vector p₀. Giventhat W_(match) is a quadratic function in terms of z, its derivativeswith respect to z are trivially obtained, and the gradient and theHessian with respect to the parameters p may be derived using the chainrule as

$\begin{matrix}{\frac{\partial W}{\partial p} = {{J^{T}\frac{\partial W}{\partial z}\mspace{14mu}{and}\mspace{14mu}\frac{\partial^{2}W}{\partial p^{2}}} = {J^{T}\frac{\partial^{2}W}{\partial z^{2}}{J.}}}} & ( {{Eq}.\mspace{11mu} 6} )\end{matrix}$

In particular embodiments, performing the matching energy optimization150 in terms of a 2D representation may lead to an under constrainedproblem, since potentially many subspace parameter configurationsp—originally deforming the surface points x—may map to the same 2D pointset z. In numerical terms, this may manifest as a large number of localminima in W_(match), which may lead to unpredictable behavior and highsensitivity of the solution to changes in the initial parameter vectorp₀. In addition, many of these local minima may express themselves assolutions where the surface representation x in 3D is highly distortedeven though the 2D points z match well. To prevent such artifacts, aregularization energy component λ·W_(reg) may be added to theoptimization problem in Eq. 4.

In particular embodiments, a parameter-based L2 regularizer ∥p−p₀∥² maybe used to favor solutions that may require the least amount of changefrom the initial pose. Depending on the specific application, employinga vertex-based regularization may be used depending on x.Parameter-based regularization may be inexpensive to include, whereasvertex-based regularization comes at a higher cost. Where the objectivefunction W also directly depends on x(p), the derivatives may need to beadjusted by adding

${J_{x}^{T}\frac{\partial W}{\partial x}\mspace{14mu}{and}\mspace{14mu} J_{x}^{T}\frac{\partial^{2}W}{\partial x^{2}}J_{x}},$respectively, where the vertex Jacobian

$J_{x} = \frac{\partial x}{\partial p}$additionally needs to be estimated using finite differences.

FIGS. 2A-2D and FIG. 3 illustrate an example of the draw-over posingembodiment. FIGS. 2A-2D illustrate an example of an embodiment of themethod applied to a rig for a character (Elephant), which is aproduction quality character rig based on a skeletal structure. FIG. 3illustrates the facial rig for a character (Mike), which is based on alarge set of blend shapes that have been sculpted individually.

In particular embodiments, draw-over posing targets on-the-flysketch-based posing of an arbitrarily rigged character. As explainedabove, to obtain a design subspace 130 suitable for draw-over posing,the user may create a custom 2D representation on the fly by placing astroke on the character. The system may store the corresponding UVvalues and may use them to reconstruct the curve after deformation bythe rig. In particular embodiments, projecting the drawn curve into thecamera plane yields its 2D representation. Once the user draws a secondcurve as a target, the system may optimize the rig's parameters to makethe two curves match.

FIG. 2A illustrates an example of the draw-over posing embodiment forinitial pose 210, focusing on Elephant's head 210A region. The solidline represents source sketch 210B, which is the first sketch a user maydraw. Source sketch 210B indicates to the system the region of thecharacter rig that the user wishes to move to a new position. The dashedline represents target sketch 210B, which is the second sketch a usermay draw. Target sketch 210C indicates to the system the position theregion selected by source sketch 210B is to be moved toward. Asillustrated in FIG. 2A, Elephant's head 210A has been moved back a smallamount to final pose 220 as a result of source sketch 210B and targetsketch 210C. Additionally, final pose 220 may alter the entirerepresentation of the elephant, since tilting back Elephant's head 210Aaffects multiple portions of the character rig.

FIG. 2B illustrates an example of the draw-over posing embodiment forinitial pose 220, focusing on Elephant's right leg 230A region. Thesolid line represents source sketch 230B, which is the first sketch auser may draw. Source sketch 230B indicates to the system the region ofthe character rig that the user wishes to move to a new position. Thedashed line represents target sketch 230B, which is the second sketch auser may draw. Target sketch 230C indicates to the system the positionthe region selected by source sketch 230B is to be moved toward. Asillustrated in FIG. 2B, Elephant's right leg 230A has been lifted up andtoward the right to final pose 240 as a result of source sketch 230B andtarget sketch 230C. Additionally, final pose 240 may alter the entirerepresentation of the elephant as lifting Elephant's right leg 230Aaffects multiple portions of the character rig.

FIG. 2C illustrates an example of the draw-over posing embodiment forinitial pose 250, focusing on Elephant's left leg 250A region. The solidline represents source sketch 250B, which is the first sketch a user maydraw. Source sketch 250B indicates to the system the region of thecharacter rig that the user wishes to move to a new position. The dashedline represents target sketch 230B, which is the second sketch a usermay draw. Target sketch 250C indicates to the system the position theregion selected by source sketch 250B is to be moved toward. Asillustrated in FIG. 2C, Elephant's left leg 250A has been lifted up tofinal pose 260 as a result of source sketch 250B and target sketch 250C.Additionally, final pose 260 may alter the entire representation of theelephant as lifting Elephant's left leg 250A affects multiple portionsof the character rig.

FIG. 2D illustrates an example of the draw-over posing embodiment forinitial pose 270, focusing on Elephant's tail 270A region. The solidline represents source sketch 270B, which is the first sketch a user maydraw. Source sketch 270B indicates to the system the region of thecharacter rig that the user wishes to move to a new position. The dashedline represents target sketch 230B, which is the second sketch a usermay draw. Target sketch 270C indicates to the system the position theregion selected by source sketch 270B is to be moved toward. Asillustrated in FIG. 2D, Elephant's tail 270A has been raised to finalpose 280 as a result of source sketch 270B and target sketch 270C.Additionally, final pose 280 may alter the entire representation of theelephant as lifting Elephant's tail 270A affects multiple portions ofthe character rig as illustrated FIG. 2D. For example, by liftingElephant's tail, the back is now arched and the belly is protruding.

FIG. 3 illustrates an example of face-posing using the draw-over posingembodiment using a facial rig for a character (Mike) based on a largeset of blend shapes. As shown in FIG. 3, initial facial expression 300Ais manipulated as shown in sketch compilation 300B, thereby resulting infinal facial expression 300C. The initial poses for right eyebrow 305and left eyebrow 310 are represented by solid sketch lines 305A and310A, which are subsequently drawn over by dashed sketch lines 305B(raising the right eyebrow to final pose 305C) and 310B (dropping theleft eyebrow to final pose 310C). The initial poses for right eyelid 315and left eyelid 320 are represented by solid sketch lines 315A and 320A,which are subsequently drawn over by dashed sketch lines 315B (droppingthe right eyelid to final pose 315C) and 320B (dropping the left eyelidto final pose 320C). The initial pose for mouth and mustache 325 isrepresented by solid sketch line 325A, which is subsequently drawn overby dashed sketch line 325B (tweaking the mouth and mustache into finalpose 325C).

In particular embodiments, small local deformations are preferred overglobal deformations. Thus, local deformations may be favored bypenalizing deformations in regions far away from the curve that is usedto generate the 2D representation. In particular embodiments, the systemmay compute the distance d of a vertex in its initial position x_(i) ¹to the curve on the mesh and penalize deviations of far away verticesvia a weighting function ƒ to obtain the energy termW _(reg)=ƒ(d(x _(i) ⁰))Σ_(i=1) ^(n)(x _(i)(p)−x _(i) ⁰)².  (Eq. 7)

Strong results may be achieved by the system when linearly blendingbetween 0 for the closest and 1 for the furthest vertex as weightingfunction ƒ. At the cost of increased runtime to minimize the matchingenergy 150, the system may optionally further improve the quality of theposing by adding a surface-based physical shell energy as a regularizer.In particular embodiments, static energy components of the discreteshells integrate directly into the system and may prevent surfacedistortions.

In particular embodiments, a user may construct the 2D characterrepresentation by embedding curves in the character structure andrigging them alongside the character's mesh; in particular embodiments,this may eliminate the need to draw source sketch lines. Projectingthese curves into the camera plane yields the 2D representation. Inparticular embodiments, the user may be required to draw the differentcurves for the target sketch in a prescribed order in order tofacilitate the correspondence computation.

Because posing a 3D character in 2D is inherently ambiguous, theseambiguities should be resolved by further regularization. To do so,motion may be restricted to the viewing plane by penalizing any movementalong the view normal n, yielding the additional vertex-based energyterm:

$\begin{matrix}{{W_{reg} = {\sum\limits_{i = 1}^{n}\;( {n \cdot ( {{x_{i}(p)} - x_{i}^{0}} )} )^{2}}},} & ( {{Eq}.\mspace{11mu} 8} )\end{matrix}$

where z_(i) ⁰ denotes the vertex position i prior to optimization.Similar to the draw-over posing embodiment, the physical shellregularization energy described above may be used to optionally improvethe posing quality.

FIG. 4 illustrates several expressive poses that may be derived fromstick-figure sketches; as shown in FIG. 4, rest pose 400A and embedded2D representation 400B may be transformed into five different posesusing sketch input. Because the 2D representation has previously beenembedded into the character structure and rigged alongside thecharacter's mesh, a user need only to draw the target sketch. Forexample, a user may want to display the figure in a seated position—tomove the figure from rest pose 400A to seated pose 410A the user maydraw sketch 410B. If the user wants to display the figure in runningpose 420A, the user may draw sketch 420B. Similarly, the user may posethe figure in pre-dive pose 430A, striding pose 440A, or lunging pose450A by drawing the corresponding sketch (430B, 440B, or 450B,respectively).

As shown in FIG. 5, to validate the redraw posing operation, a CartoonMan character is used which includes embedded line representations forall body parts. FIG. 5 depicts an animation 500 of the Cartoon Manfalling backward. The Cartoon Man starts in rest pose 510 and aftereight sketch inputs transforming 2D representation 515, the Cartoon Manhas fallen backward and landed on his back in final pose 520.

In particular embodiments, a user may make fine-scale adjustments to arig as shown in FIG. 6. A user may first select a specific location onor region of the character rig to manipulate (e.g., the character'sneck). Upon receiving the selection, the system may display a controlelement (e.g., control wheel 610), enabling the user to manipulate therig into the desired position (e.g., by turning control wheel 610 tocause the head and neck to tilt).

In particular embodiments, the system may enable sketch-based characterindividualization by allowing users to design a virtual character basedon simple sketches and predefined adaptive model parts that may exposerig controls that affect the shape in addition to the pose. Sketch-basedcharacter individualization enables the user to design individualcharacter parts offline, which can then be easily combined and posedinteractively using sketch input as illustrated in FIG. 6.

FIG. 7 illustrates an example application of sketch-based characterindividualization for a character comprising parts of a dragon modelthat have been rigged: torso 710A, wings 705A, head 715A, tail 720A,front legs 740A and hind legs 730A. In addition to the correspondingmesh, each part of the dragon model also contains an embedded strokegesture that serves as the 2D representation to be deformed by the rigand as an identifier to help determine classification of the drawn part.For example, to instantiate torso 710A, a user must sketch oval 710B.Similarly, in order to instantiate wing 705A, head 715A, tail 720A,front legs 740A, or hind legs 730A, a user must sketch triangle 705B,head 715B, tail 720B, front legs 740B, or hind legs 730B, respectively,as shown in FIG. 7.

In particular embodiments, to allow the parts to match a broad range ofuser-drawn shapes, the rigs may expose the local scaling parameters ofthe underlying bones used to skin the models in addition to typicalposing controls such as rigid transformation and joint rotations.Instead lieu of imposing a drawing order or requiring the user tospecify which body part he or she is drawing, the system may use adatabase of 2D sketches containing several example instances of eachbody part together with a category recognition approach to classify, andthereby detect, each drawn stroke.

In particular embodiments, the system then may instantiate the detectedpart and optimize it for both its shape and pose parameters (in someembodiments, simultaneously) to place it into the scene. FIG. 8illustrates results using a character individualization method asdescribed herein. With reference to FIG. 8, in order to create a torsopart, a user may be able to sketch a large oval (810A), small oval(810B), or an elongated and saggy oval (810C), which are detected,instantiated, and then optimized as a large torso (820A), a small torso(820B), or an elongated and saggy torso (820C). Also, as shown in FIG.8, a user may be able to sketch a variable number of foreleg sketches(830A-830C) or hindleg sketches (850A-850C), which are detected,instantiated, and then optimized in the appropriate number, shape, andpose as foreleg parts 840A-840C and hindleg parts 860A-860C; as can beseen in the middle character in FIG. 8, in particular embodiments,sketches of a single foreleg 830B and a single hindleg 850B may beinstantiated as a pair of foreleg parts 840B and a pair of hindleg parts860B. As further shown in FIG. 8, sketches 870A-870C may be similarlydetected, instantiated, and then optimized in the appropriate number,shape, and pose as wings 880A-880C.

In particular embodiments, after reviewing the character's 3Dinstantiation, the user may either redraw any stroke and repeat thefitting of the corresponding part, or continue to add new parts to thecharacter.

In particular embodiments, the design spaces explored by the method mayeither be explicitly provided by artists through parametrized rigs, orthey can be implicitly defined. In particular embodiments, both themechanical structure and the motions of a complex mechanical charactermay be specified using techniques as described herein. Such mechanicalassemblies may be modeled using a collection of components andconstraints.

In 2D, the configuration of each component i is given by its states_(i)=α, x; y)^(T), where α describes the component's orientation, and xand y hold its world-space position. The vector s aggregates the statesof all components in a mechanical assembly. Constraints C (s) areintroduced to restrict the relative motion between pairs of components,and they are used to model virtual motors and different types ofmechanical joints. To solve for the motion of the assembly, the systemcomputes the state vector s that minimizes ½C(s)^(T)C(s) using Newton'smethod.

In particular embodiments, the system may receive as input a library ofparametrized mechanisms that represent, for example, different types oflimbs that a mechanical character might have. As illustrated in FIG. 9,skeletal curves may be rigidly attached to the main mechanical structureof the mechanisms. These skeletal curves may be used to differentiatebetween the different classes of mechanism that are available, and todefine the energy term introduced above. FIG. 9 illustrates mechanicalleg 910B and its corresponding 2D representation 910A. Additionally,mechanical leg 910B may be assigned a motion curve 920 which determinesthe range of motion for mechanical leg 910B. The world coordinates forany point u along a skeletal curve are computed as z(p)=R_(i)u+t_(i),where R_(i) is the orientation, and t_(i) the translation of thecomponent the curve is attached to. The parameters p may be optimized toallow the system to simultaneously change a mechanisms' kinematicproperties (e.g., the length of any rigid segment) and motion; inparticular embodiments, they may consist of the local coordinates thatdefine the location of each joint.

As illustrated in FIG. 10, a design system to generate two types of legsand combine them with a rigid body shape parameterized in translation,rotation and scale along the x and y axes is presented. A user may firstsketch mechanical body 1010A of any size, which results in displayingits corresponding display body 1010B. Additionally, a user may thensketch mechanical leg 1020A and motion curves 1030 resulting in acomplete mechanical assembly with a range of motion defined in themechanical legs 1020B. Using these models as input, the user may createa customized mechanically simulated beast. As further illustrated by theversion of the beast at the bottom of FIG. 10, using the techniquesdescribed herein, the user may adapt the shape of the legs, their motionpaths, or the relative phase between them may be done quickly andintuitively.

FIG. 11A illustrates a flowchart for an example method 1100A ofsketch-based posing of 3D characters. The method may begin at step1110A, where particular embodiments may provide a 2D visualrepresentation of an object to be animated. At step 1120A, particularembodiments may receive sketch input with respect to the object, whereinthe sketch input identifies a target position for a specified portion ofthe object. At step 1130A, particular embodiments may minimize an energymetric by incorporating aspects of the character rig specification asunknown values. At step 1140A, particular embodiments may calculate theclosest point between the original position of the specified portion andthe updated position of the specified portion. At step 1150A, particularembodiments may compute a deformation for the object. Particularembodiments may repeat one or more steps of the method of FIG. 11A,where appropriate. Although this disclosure describes and illustratesparticular steps of the method of FIG. 11A as occurring in a particularorder, this disclosure contemplates any suitable steps of the method ofFIG. 11A occurring in any suitable order. Moreover, although thisdisclosure describes and illustrates an example method for sketch-basedposing of 3D characters, including the particular steps of the method ofFIG. 11A, this disclosure contemplates any suitable method forsketch-based posing of 3D characters, including any suitable steps,which may include all, some, or none of the steps of the method of FIG.11A, where appropriate. Furthermore, although this disclosure describesand illustrates particular components, devices, or systems carrying outparticular steps of the method of FIG. 11A, this disclosure contemplatesany suitable combination of any suitable components, devices, or systemscarrying out any suitable steps of the method of FIG. 11A.

FIG. 11B illustrates a flowchart for an example method 1100B of designand customization of 3D characters using sketch-based techniques. Themethod may begin at step 1110B, where particular embodiments may providea plurality of objects, wherein each of the objects comprises a 2Drepresentation and a 3D model. At step 1120B, particular embodiments maydetect sketch input. At step 1130B, particular embodiments may classifythe sketch input as corresponding to a detected one of the objects. Atstep 1140B, particular embodiments may instantiate the detected object.At step 1150B, particular embodiments may display a 3D visualrepresentation of the instantiated object. Particular embodiments mayrepeat one or more steps of the method of FIG. 11B, where appropriate.Although this disclosure describes and illustrates particular steps ofthe method of FIG. 11B as occurring in a particular order, thisdisclosure contemplates any suitable steps of the method of FIG. 11Boccurring in any suitable order. Moreover, although this disclosuredescribes and illustrates an example method for design and customizationof 3D characters using sketch-based techniques, including the particularsteps of the method of FIG. 11B, this disclosure contemplates anysuitable method for design and customization of 3D characters usingsketch-based techniques, including any suitable steps, which may includeall, some, or none of the steps of the method of FIG. 11B, whereappropriate. Furthermore, although this disclosure describes andillustrates particular components, devices, or systems carrying outparticular steps of the method of FIG. 11B, this disclosure contemplatesany suitable combination of any suitable components, devices, or systemscarrying out any suitable steps of the method of FIG. 11B.

FIG. 12 illustrates an example computer system 1200. In particularembodiments, one or more computer systems 1200 perform one or more stepsof one or more methods described or illustrated herein. In particularembodiments, one or more computer systems 1200 provide functionalitydescribed or illustrated herein. In particular embodiments, softwarerunning on one or more computer systems 1200 performs one or more stepsof one or more methods described or illustrated herein or providesfunctionality described or illustrated herein. Particular embodimentsinclude one or more portions of one or more computer systems 1200.Herein, reference to a computer system may encompass a computing device,and vice versa, where appropriate. Moreover, reference to a computersystem may encompass one or more computer systems, where appropriate.

This disclosure contemplates any suitable number of computer systems1200. This disclosure contemplates computer system 1200 taking anysuitable physical form. As example and not by way of limitation,computer system 1200 may be an embedded computer system, asystem-on-chip (SOC), a single-board computer system (SBC) (such as, forexample, a computer-on-module (COM) or system-on-module (SOM)), adesktop computer system, a laptop or notebook computer system, aninteractive kiosk, a mainframe, a mesh of computer systems, a mobiletelephone, a personal digital assistant (PDA), a server, a tabletcomputer system, or a combination of two or more of these. Whereappropriate, computer system 1200 may include one or more computersystems 1200; be unitary or distributed; span multiple locations; spanmultiple machines; span multiple data centers; or reside in a cloud,which may include one or more cloud components in one or more networks.Where appropriate, one or more computer systems 1200 may perform withoutsubstantial spatial or temporal limitation one or more steps of one ormore methods described or illustrated herein. As an example and not byway of limitation, one or more computer systems 1200 may perform in realtime or in batch mode one or more steps of one or more methods describedor illustrated herein. One or more computer systems 1200 may perform atdifferent times or at different locations one or more steps of one ormore methods described or illustrated herein, where appropriate.

In particular embodiments, computer system 1200 includes a processor1202, memory 1204, storage 1206, an input/output (I/O) interface 1208, acommunication interface 1210, and a bus 1212. Although this disclosuredescribes and illustrates a particular computer system having aparticular number of particular components in a particular arrangement,this disclosure contemplates any suitable computer system having anysuitable number of any suitable components in any suitable arrangement.

In particular embodiments, processor 1202 includes hardware forexecuting instructions, such as those making up a computer program. Asan example and not by way of limitation, to execute instructions,processor 1202 may retrieve (or fetch) the instructions from an internalregister, an internal cache, memory 1204, or storage 1206; decode andexecute them; and then write one or more results to an internalregister, an internal cache, memory 1204, or storage 1206. In particularembodiments, processor 1202 may include one or more internal caches fordata, instructions, or addresses. This disclosure contemplates processor1202 including any suitable number of any suitable internal caches,where appropriate. As an example and not by way of limitation, processor1202 may include one or more instruction caches, one or more datacaches, and one or more translation lookaside buffers (TLBs).Instructions in the instruction caches may be copies of instructions inmemory 1204 or storage 1206, and the instruction caches may speed upretrieval of those instructions by processor 1202. Data in the datacaches may be copies of data in memory 1204 or storage 1206 forinstructions executing at processor 1202 to operate on; the results ofprevious instructions executed at processor 1202 for access bysubsequent instructions executing at processor 1202 or for writing tomemory 1204 or storage 1206; or other suitable data. The data caches mayspeed up read or write operations by processor 1202. The TLBs may speedup virtual-address translation for processor 1202. In particularembodiments, processor 1202 may include one or more internal registersfor data, instructions, or addresses. This disclosure contemplatesprocessor 1202 including any suitable number of any suitable internalregisters, where appropriate. Where appropriate, processor 1202 mayinclude one or more arithmetic logic units (ALUs); be a multi-coreprocessor; or include one or more processors 1202. Although thisdisclosure describes and illustrates a particular processor, thisdisclosure contemplates any suitable processor.

In particular embodiments, memory 1204 includes main memory for storinginstructions for processor 1202 to execute or data for processor 1202 tooperate on. As an example and not by way of limitation, computer system1200 may load instructions from storage 1206 or another source (such as,for example, another computer system 1200) to memory 1204. Processor1202 may then load the instructions from memory 1204 to an internalregister or internal cache. To execute the instructions, processor 1202may retrieve the instructions from the internal register or internalcache and decode them. During or after execution of the instructions,processor 1202 may write one or more results (which may be intermediateor final results) to the internal register or internal cache. Processor1202 may then write one or more of those results to memory 1204. Inparticular embodiments, processor 1202 executes only instructions in oneor more internal registers or internal caches or in memory 1204 (asopposed to storage 1206 or elsewhere) and operates only on data in oneor more internal registers or internal caches or in memory 1204 (asopposed to storage 1206 or elsewhere). One or more memory buses (whichmay each include an address bus and a data bus) may couple processor1202 to memory 1204. Bus 1212 may include one or more memory buses, asdescribed below. In particular embodiments, one or more memorymanagement units (MMUs) reside between processor 1202 and memory 1204and facilitate accesses to memory 1204 requested by processor 1202. Inparticular embodiments, memory 1204 includes random access memory (RAM).This RAM may be volatile memory, where appropriate Where appropriate,this RAM may be dynamic RAM (DRAM) or static RAM (SRAM). Moreover, whereappropriate, this RAM may be single-ported or multi-ported RAM. Thisdisclosure contemplates any suitable RAM. Memory 1204 may include one ormore memories 1204, where appropriate. Although this disclosuredescribes and illustrates particular memory, this disclosurecontemplates any suitable memory.

In particular embodiments, storage 1206 includes mass storage for dataor instructions. As an example and not by way of limitation, storage1206 may include a hard disk drive (HDD), a floppy disk drive, flashmemory, an optical disc, a magneto-optical disc, magnetic tape, or aUniversal Serial Bus (USB) drive or a combination of two or more ofthese. Storage 1206 may include removable or non-removable (or fixed)media, where appropriate. Storage 1206 may be internal or external tocomputer system 1200, where appropriate. In particular embodiments,storage 1206 is non-volatile, solid-state memory. In particularembodiments, storage 1206 includes read-only memory (ROM). Whereappropriate, this ROM may be mask-programmed ROM, programmable ROM(PROM), erasable PROM (EPROM), electrically erasable PROM (EEPROM),electrically alterable ROM (EAROM), or flash memory or a combination oftwo or more of these. This disclosure contemplates mass storage 1206taking any suitable physical form. Storage 1206 may include one or morestorage control units facilitating communication between processor 1202and storage 1206, where appropriate. Where appropriate, storage 1206 mayinclude one or more storages 1206. Although this disclosure describesand illustrates particular storage, this disclosure contemplates anysuitable storage.

In particular embodiments, I/O interface 1208 includes hardware,software, or both, providing one or more interfaces for communicationbetween computer system 1200 and one or more I/O devices. Computersystem 1200 may include one or more of these I/O devices, whereappropriate. One or more of these I/O devices may enable communicationbetween a person and computer system 1200. As an example and not by wayof limitation, an I/O device may include a keyboard, keypad, microphone,monitor, mouse, printer, scanner, speaker, still camera, stylus, tablet,touch screen, trackball, video camera, another suitable I/O device or acombination of two or more of these. An I/O device may include one ormore sensors. This disclosure contemplates any suitable I/O devices andany suitable I/O interfaces 1208 for them. Where appropriate, I/Ointerface 1208 may include one or more device or software driversenabling processor 1202 to drive one or more of these I/O devices. I/Ointerface 1208 may include one or more I/O interfaces 1208, whereappropriate. Although this disclosure describes and illustrates aparticular I/O interface, this disclosure contemplates any suitable I/Ointerface.

In particular embodiments, communication interface 1210 includeshardware, software, or both providing one or more interfaces forcommunication (such as, for example, packet-based communication) betweencomputer system 1200 and one or more other computer systems 1200 or oneor more networks. As an example and not by way of limitation,communication interface 1210 may include a network interface controller(NIC) or network adapter for communicating with an Ethernet or otherwire-based network or a wireless NIC (WNIC) or wireless adapter forcommunicating with a wireless network, such as a WI-FI network. Thisdisclosure contemplates any suitable network and any suitablecommunication interface 1210 for it. As an example and not by way oflimitation, computer system 1200 may communicate with an ad hoc network,a personal area network (PAN), a local area network (LAN), a wide areanetwork (WAN), a metropolitan area network (MAN), or one or moreportions of the Internet or a combination of two or more of these. Oneor more portions of one or more of these networks may be wired orwireless. As an example, computer system 1200 may communicate with awireless PAN (WPAN) (such as, for example, a BLUETOOTH WPAN), a WI-FInetwork, a WI-MAX network, a cellular telephone network (such as, forexample, a Global System for Mobile Communications (GSM) network), orother suitable wireless network or a combination of two or more ofthese. Computer system 1200 may include any suitable communicationinterface 1210 for any of these networks, where appropriate.Communication interface 1210 may include one or more communicationinterfaces 1210, where appropriate. Although this disclosure describesand illustrates a particular communication interface, this disclosurecontemplates any suitable communication interface.

In particular embodiments, bus 1212 includes hardware, software, or bothcoupling components of computer system 1200 to each other. As an exampleand not by way of limitation, bus 1212 may include an AcceleratedGraphics Port (AGP) or other graphics bus, an Enhanced Industry StandardArchitecture (EISA) bus, a front-side bus (FSB), a HYPERTRANSPORT (HT)interconnect, an Industry Standard Architecture (ISA) bus, an INFINIBANDinterconnect, a low-pin-count (LPC) bus, a memory bus, a Micro ChannelArchitecture (MCA) bus, a Peripheral Component Interconnect (PCI) bus, aPCI-Express (PCIe) bus, a serial advanced technology attachment (SATA)bus, a Video Electronics Standards Association local (VLB) bus, oranother suitable bus or a combination of two or more of these. Bus 1212may include one or more buses 1212, where appropriate. Although thisdisclosure describes and illustrates a particular bus, this disclosurecontemplates any suitable bus or interconnect.

Herein, a computer-readable non-transitory storage medium or media mayinclude one or more semiconductor-based or other integrated circuits(ICs) (such, as for example, field-programmable gate arrays (FPGAs) orapplication-specific ICs (ASICs)), hard disk drives (HDDs), hybrid harddrives (HHDs), optical discs, optical disc drives (ODDs),magneto-optical discs, magneto-optical drives, floppy diskettes, floppydisk drives (FDDs), magnetic tapes, solid-state drives (SSDs),RAM-drives, SECURE DIGITAL cards or drives, any other suitablecomputer-readable non-transitory storage media, or any suitablecombination of two or more of these, where appropriate. Acomputer-readable non-transitory storage medium may be volatile,non-volatile, or a combination of volatile and non-volatile, whereappropriate.

Herein, “or” is inclusive and not exclusive, unless expressly indicatedotherwise or indicated otherwise by context. Therefore, herein, “A or B”means “A, B, or both,” unless expressly indicated otherwise or indicatedotherwise by context. Moreover, “and” is both joint and several, unlessexpressly indicated otherwise or indicated otherwise by context.Therefore, herein, “A and B” means “A and B, jointly or severally,”unless expressly indicated otherwise or indicated otherwise by context.

The scope of this disclosure encompasses all changes, substitutions,variations, alterations, and modifications to the example embodimentsdescribed or illustrated herein that a person having ordinary skill inthe art would comprehend. The scope of this disclosure is not limited tothe example embodiments described or illustrated herein. Moreover,although this disclosure describes and illustrates respectiveembodiments herein as including particular components, elements,functions, operations, or steps, any of these embodiments may includeany combination or permutation of any of the components, elements,functions, operations, or steps described or illustrated anywhere hereinthat a person having ordinary skill in the art would comprehend.Furthermore, reference in the appended claims to an apparatus or systemor a component of an apparatus or system being adapted to, arranged to,capable of, configured to, enabled to, operable to, or operative toperform a particular function encompasses that apparatus, system,component, whether or not it or that particular function is activated,turned on, or unlocked, as long as that apparatus, system, or componentis so adapted, arranged, capable, configured, enabled, operable, oroperative.

What is claimed is:
 1. A method, performed by one or more computingdevices, comprising: providing a two-dimensional (2D) representation ofan object to be animated, wherein the 2D representation of the object iscreated by projection of a three-dimensional (3D) model of the objectinto a viewing plane; mapping rig parameters of a character rigspecification for the object to a 2D representation of the 3D model andto a surface mesh for the 3D model, wherein changes to the character rigspecification simultaneously influence the surface mesh and the 2Drepresentation; receiving sketch input with respect to the object,wherein the sketch input identifies a target position for a specifiedportion of the object; updating the character rig specification based onthe target position for the specified portion of the object; computing adeformation for the object based on the updated character rigspecification, wherein: the surface mesh and the 2D representation aredeformed in accordance with the computed deformation, the character rigspecification comprises a defined space of deformations, and thecomputed deformation is within the defined space; and displaying anupdated version of the 2D representation of the object based on thecomputed deformation.
 2. The method of claim 1, wherein the sketch inputfor the target position is drawn over the 2D representation of theobject.
 3. The method of claim 1, wherein the deformation for the objectis computed based on a nonlinear iterative closest point calculationbetween an original position of the specified portion and an updatedposition of the specified portion.
 4. The method of claim 3, wherein theupdated position of the specified portion is determined by sampling thesketch input identifying the target position.
 5. The method of claim 3,wherein the nonlinear iterative closest point calculation incorporatescorrespondence weights determined between a first plurality of pointsalong the 2D representation of the object and a second plurality ofpoints along the sketch input identifying the target position.
 6. Themethod of claim 3, wherein the nonlinear iterative closest pointcalculation is based on minimizing an energy metric by incorporatingaspects of the character rig specification as unknown values.
 7. Themethod of claim 6, wherein the minimizing the energy metric incorporatesa regularization energy component.
 8. The method of claim 1, furthercomprising receiving initial sketch input comprising a curve drawn overa mesh for the object, wherein the 2D representation of the object isbased on the initial sketch input, and wherein the curve is incorporatedin the computed deformation.
 9. The method of claim 8, wherein thecomputing the deformation comprises penalizing any deformation inregions remote from the curve of the initial sketch input.
 10. Themethod of claim 1, wherein the 2D representation of the object is basedon a set of curves embedded into the object, and wherein the curves arerigged alongside a mesh for the object.
 11. One or morecomputer-readable non-transitory storage media embodying software thatis operable when executed to: provide a two-dimensional (2D)representation of an object to be animated, wherein the 2Drepresentation of the object is created by projection of athree-dimensional (3D) model of the object into a viewing plane; map rigparameters of a character rig specification for the object to a 2Drepresentation of the 3D model and to a surface mesh for the 3D model,wherein changes to the character rig specification simultaneouslyinfluence the surface mesh and the 2D representation; receive sketchinput with respect to the object, wherein the sketch input identifies atarget position for a specified portion of the object; update thecharacter rig specification based on the target position for thespecified portion of the object; compute a deformation for the objectbased on the updated character rig specification for the object, whereinthe character rig specification comprises the 3D model of the object,and wherein: the surface mesh and the 2D representation are deformed inaccordance with the computed deformation, the character rigspecification comprises a defined space of deformations, and thecomputed deformation is within the defined space; and display an updatedversion of the 2D representation of the object based on the computeddeformation.
 12. The media of claim 11, wherein the sketch input for thetarget position is drawn over the 2D representation of the object. 13.The media of claim 11, wherein the deformation for the object iscomputed based on a nonlinear iterative closest point calculationbetween an original position of the specified portion and an updatedposition of the specified portion.
 14. The media of claim 13, whereinthe updated position of the specified portion is determined by samplingthe sketch input identifying the target position.
 15. The media of claim13, wherein the nonlinear iterative closest point calculationincorporates correspondence weights determined between a first pluralityof points along the 2D representation of the object and a secondplurality of points along the sketch input identifying the targetposition.
 16. The media of claim 13, wherein the nonlinear iterativeclosest point calculation is based on minimizing an energy metric byincorporating aspects of the character rig specification as unknownvalues.
 17. The media of claim 16, wherein the minimizing the energymetric incorporates a regularization energy component.
 18. The media ofclaim 11, wherein the software is further operable when executed toreceive initial sketch input comprising a curve drawn over a mesh forthe object, wherein the 2D representation of the object is based on theinitial sketch input, and wherein the curve is incorporated in thecomputed deformation.
 19. The media of claim 18, wherein the softwarethat is operable when executed to compute the deformation comprisessoftware that is operable when executed to penalize any deformation inregions remote from the curve of the initial sketch input.
 20. A systemcomprising: one or more processors; and one or more computer-readablenon-transitory storage media coupled to one or more of the processorsand comprising instructions operable when executed by one or more of theprocessors to cause the system to: provide a two-dimensional (2D)representation of an object to be animated, wherein the 2Drepresentation of the object is created by projection of athree-dimensional (3D) model of the object into a viewing plane; map rigparameters of a character rig specification for the object to a 2Drepresentation of the 3D model and to a surface mesh for the 3D model,wherein changes to the character rig specification simultaneouslyinfluence the 3D model and the 2D representation of the 3D model;receive sketch input with respect to the object, wherein the sketchinput identifies a target position for a specified portion of theobject; update the character rig specification based on the targetposition for the specified portion of the object; compute a deformationfor the object based on the updated character rig specification for theobject, wherein the character rig specification comprises the 3D modelof the object, and wherein: the surface mesh and the 2D representationare deformed in accordance with the computed deformation, the characterrig specification comprises a defined space of deformations, and thecomputed deformation is within the defined space; and display an updatedversion of the 2D representation of the object based on the computeddeformation.